(1) Field of the Invention
The present invention relates to a fat-desensitizing for a litho printing plate. In particular, the present invention relates to a liquid composition useful for a fat-desensitizing treatment for a litho printing plate comprising an electroconductive substrate plate and an electrophotographic photosensitive layer thereon.
The term "fat-desensitizing" refers to a desensitization of non-image formed portions in a litho printing plate face for printing ink for lithography.
(2) Description of the Related Art
It is known that a litho printing plate having printing images formed on a printing plate face by electrophotography is composed of a sheet substrate and a photoconductive layer containing, as a principal component, a photoconductive substance, for example, zinc oxide.
In a method for the preparation of the printing images, a photosensitive layer on a litho printing plate is exposed to light through a negative or positive mask having a desired pattern of images and the photosensitive layer is developed with a commercially available toner.
In another method for producing an offset litho printing plate, a printing base plate is prepared by forming an image-receiving layer comprising an inorganic pigment and a resinous binder on a surface of a sheet substrate, electrophotographic images are separately formed on a photoconductive transfer drum, for example, a selenic drum, and the images on the drum are transferred to the image-receiving layer.
In a still another method for an offset litho printing plate, desired images are formed on an image-receiving layer in a printing base plate by hand-writing or typing with an oil paint. This method is the so-called direct image-formed offset masterproducing method.
In the offset litho printing plate, the printing plate face has image-formed portions and non-image-formed portions thereof. The non-image-formed portion must be hydrophilic and, therefore, a fat-desensitizing treatment must be applied to the non-image-formed portion. Paricularly, in the litho printing plate having a photoconductive layer, since the non-image-formed portions of the photosensitive layer must be primarily hydrophilic, but usually exhibit a considerably intense lipophilic property, the fat-desensitizing treatment must be applied to the non-image-formed portions of the printing plate face.
If the fat-desensitizing treatment is insufficiently carried out, the resultant non-image-formed portions of the printing plate face are stained during the printing procedures. Especially, where the printing procedures are continued, over a long period of time, the stains on the non-image-formed portions of the printing plate face make it impossible to stably produce clear prints free from stains.
As the fat-desensitization treating liquid, i.e., an etching liquid for the printing plate face, the following liquids are known:
(1) Treating liquids containing, as a principal component, at least a salt selected from organic acid salts and inorganic acid salts, as disclosed in Japanese Examined Patent Publication No. 43-28404.
(2) Treating liquids containing, as a principal component, at least a member selected from ferrocyanide salts and ferricyanide salts, as disclosed in Japanese Examined Patent Publication No. 39-8416.
(3) Treating liquid containing, as a principal component, phytic acid, as disclosed in Japanese Examined Patent Publication No. 45-24609.
The inorganic or organic salt treating liquid (1) is disadvantageous in that it has a low fat-desensitizing effect and, therefore, stains are formed on the resultant prints, and is unsatisfactory when attempting to provide a litho printing plate capable of continuously producing a number of clear prints over a long period of time.
The ferrocyanide or ferricyanide salt-containing fat-desensitizing liquid (2) exhibits a greater fat-desensitizing effect than that of the inorganic or organic salt-containing fat desensitizing liquid (1), but the level of the effect is still unsatisfactory. Therefore, when the fat-desensitizing liquid (2) is utilized for a printing process for neutral paper, which frequently generates paper powder, or for a printing procedure under a high printing pressure, printing stains are easily generated on the resultant prints. The fat-desensitizing liquid (2) is also disadvantageous in that it has a poor stability to heat and light.
Also, the fat-desensitizing liquid (2) contains cyan ion (CN.sup.-), which is toxic to the human body. Usually, the ferrocyan ion (Fe(CN).sub.6).sup.4- and the ferricyan ion (Fe(CN).sub.6).sup.3- are chemically stable and harmless to the human body. However, the ferrocyan ion and ferrcyan ions could be decompose and be converted to the toxic cyan ions under certain environmental conditions. Accordingly, the fat-desensitizing liquid (2) must be used with the greatest circumspection, to prevent a chemical decomposition of the ferrocyan or ferricyan ion.
In order to eliminate the above-mentioned disadvantages of the fat-desensitizing liquids (1) and (2), the phytic acid- containing fat-desensitizing liquid (3) was provided. However, the fat-desensitizing liquid (3) exhibits a poor chelating property and an unsatisfactory fat-desensitizing effect and, therefore, cannot be industrially utilized.
In consideration of the above-mentioned circumstances, there is a strong demand for the provision of a new fat- desensitizing composition free from the above-mentioned disadvantages.
Because phytic acid and its functional derivatives are nonpoisonous and harmless to the human body, but per se exhibit an unsatisfactory fat-desensitizing activity, they are believed to be useless as a fat-desensitizing agent. Nevertheless, the inventors of the present invention have attempted to utilize them as a component for an industrial useful fat-desensitizing composition.